This project is designed to accomplish the following three objectives: (1) Isolation and characterization of T cell subpopulations using physical and functional criteria; (2) Evaluation of the functional capacity of isolated T cell subpopulations in various in vivo and in vitro immune reactions; (3) Elucidation of the nature of T cell synergy in the Mixed Lymphocyte Interaction (MLI) and Graft verus Host Reaction (GVHR).